The demand for smaller and multifunction electronic devices such as wireless phones with internet access, PDA, etc. has necessitated an ever increasing density of multilayer circuits. These multilayer circuits typically use dielectric materials which act as insulating substrates onto which are deposited conducting metals to form a wiring layer.
Very early on, alumina was the dielectric material of choice, which because of its high sintering temperature (1400°-1500° C.) necessitated the use of metals such as tungsten or molybdenum. However, as signal transmission speeds have increased to 1 GHz and above, the use of tungsten or molybdenum wire is not possible because of their high resistivity. This necessitates the use of lower resistivity metals such as gold, silver or copper. These metals, however, have much lower melting temperatures which in turn require dielectrics which sinter at temperature as low as 900° C.
The art has attempted to meet this need with low temperature cofired ceramics (LTCC). These ceramics are formed by mixing, glassy materials with crystalline phases. For example, U.S. Pat. No. 5,821,181 discloses forming a mixture of alumina, a glassy precursor material and a modifier. The glassy material contains SiO2, B2O3, at least one of MgO, CaO, SrO and BaO and at least one of K2O, Na2O and Li2O; while the modifier is selected from TiO2, SrTiO3 and CaTiO3. U.S. Pat. No. 4,714,687 discloses preparing a glass-ceramic containing willemite as the crystalline phase. The glass-ceramic body contains ZnO, MgO, Al2O3 and SiO2. U.S. Pat. No. 5,164,342 discloses a low dielectric glass ceramic containing CaO, B2O3 and SiO2. In U.S. Pat. No. 6,232,251 B1 a dielectric ceramic is disclosed which comprises a diopside oxide crystal phase; a crystal phase selected from a quartz crystal phase and a composite oxide crystal phase containing Ti and Mg or Zn and a glass phase.
S. M. Clark et al in Nonequil. Phenom. Supercooled Fluids, Glasses Amorphous Mater., Proc. Workshop, 1996, 241-242, describe using zinc oxide in combination with a glass to form cordierite. Similarly P. S. Rogers and J. Williamson in Proceedings from the IX International Congress on Glass, Versailles, France, Sep. 27 to 2 Oct. 1971, section A1.3, 411-416, disclose that mixing zinc oxide with glass increases the crystal growth rate. G. Sankar et al in J. Phys. Chem. 1993, 97, 9550-9554 disclose that adding zinc oxide to MgB zeolite lowers the temperature at which cordierite forms by 100° C.
It is also disclosed in the art that zeolitic compositions can be sintered directly into ceramics. Thus, U.S. Pat. No. 5,071,801 discloses preparing a high density leucite based ceramic from a potassium, cesium or rubidium exchanged zeolite. The zeolite has a SiO2/Al2O3 ratio of 3.5 to about 7.5. Similarly U.S. Pat. No. 4,980,323 and U.S. Pat. No. 5,064,790 disclose a process and a cordierite ceramic prepared by the process. The cordierite can be prepared from a Mg exchanged zeolite such as zeolite B, phillipsite, harmotome, gismondine, zeolite ZK-19 and zeolite W. Further, U.S. Pat. No. 5,036,030 discloses a process for preparing an alkaline earth metal aluminosilicate sintered body from a mixture of a zeolite in the alkaline earth metal form, having a SiO2/Al2O3 ratio of not more than 3.0 and a boron compound such as boron oxide. Finally, U.S. Pat. No. 5,166,107 discloses preparing an anorthite sintered body from a calcium type zeolite having a SiO2/Al2O3 ratio of less that 3.0.
In contrast to this art, applicant has prepared a dielectric ceramic article from a mixture of a crystalline aluminosilicate zeolite, a glass phase and zinc oxide. The advantage to the use of zeolites in forming dielectric ceramic articles is that the higher reactivity of zeolite-derived powders vs. crystalline ceramic fillers such as alumina and cordierite allows more facile and complete solid state reaction during sintering and therefore easier formation of desired high-Q phases. The pressure of zinc oxide also lowers the time and temperature needed for adequate sintering and crystallization of cordierite. Other advantages include increased uniformity in the resulting composition and associated increased uniformity in its electrical properties.